The present invention relates to a semiconductor electric power converter which uses semiconductor elements and the like and more particularly relates to a semiconductor electric power converter which suppresses overvoltage during a switching operation.
In an IGBT power converter, when an IGBT is turned off, the energy stored in a line while the IGBT has been on becomes a surge voltage and is applied to said IGBT. Japanese Application Patent Laid-open Publication No. Hei.10-248237 has disclosed a method of preventing elements from being damaged due to the application of overvoltage including a surge voltage generated when an IGBT is turned off. This method employs a capacitor connected between a gate and a collector of the IGBT to suppress a voltage rise rate (dv/dt) of the collector.
Normally, to turn off an IGBT, a switch that connects a gate terminal and a negative potential line of a gate circuit is closed. When a switch located between a gate terminal and a negative potential line of a gate circuit is closed, an electric charge that has been charged in the gate is drawn to the gate circuit and a gate voltage is decreased. As a gate voltage decreases, a collector voltage increases and the IGBT enters into the OFF state from the ON state. If a capacitor is connected between the IGBT collector and the gate thereof, a current is supplied to the gate in proportion to dv/dt of the collector voltage. This restrains a decrease in gate voltage or increases a gate voltage, thus decreasing the dv/dt of the collector voltage and protecting the collector from the application of overvoltage. FIG. 2 illustrates the main part of one arm of a power converter in accordance with the prior art.
If a dv/dt of the collector voltage is high, an excessive gate current is supplied by a capacitor connected between a collector and an emitter of an IGBT, which excessively increases a gate voltage and decreases the impedance of the IGBT more than necessary. This excessive decrease in impedance of the IGBT results in increasing unnecessary turn-off loss. Further, when a collector voltage decreases due to an excessive supply of gate current, said capacitor draws the gate current from the IGBT gate and a gate voltage rapidly decreases. As a gate voltage rapidly decreases, a collector voltage quickly increases, which causes the collector voltage and gate voltage vibration problems.
In view of the foregoing, the main purpose of the present invention is to provide a power converter which employs a circuit that prevents the collector voltage from vibrating and also prevents the application of overvoltage to the collector by minimizing an increase in loss of the IGBT.
A semiconductor electric power converter in accordance with the present invention comprises a capacitor which is connected between a collector and a gate of an IGBT and means for immediately discharging an electric charge from the gate when a gate voltage increases more than a gate voltage command value specified in response to the collector voltage. This configuration allows the semiconductor electric power converter in accordance with the present invention to control an appropriate gate voltage, which prevents the collector voltage from vibrating and also minimizes an increase in loss of the IGBT to clamp the collector voltage.
A semiconductor electric power converter in accordance with the present invention employs means for immediately discharging an electric charge stored in a gate when a gate voltage exceeds a gate voltage command value specified in response to the collector voltage. The means include means for decreasing the impedance between a gate terminal and a gate circuit of the IGBT and means for decreasing the impedance between the gate and an emitter when a collector voltage has been increased.
In a semiconductor electric power converter in accordance with the present invention, a capacitor connected between a collector and a gate of an IGBT supplies a gate current to the IGBT gate to increase the gate voltage when a dv/dt of the collector voltage is high. This protects the IGBT from the application of overvoltage. Further, in a semiconductor electric power converter in accordance with the present invention, when a gate voltage of an IGBT is higher than a gate voltage command value specified in response to the collector voltage, the impedance between either the gate and the gate circuit or between the gate and the emitter is decreased so that a current supplied by the capacitor connected between the collector and the emitter can be bypassed due to the impedance between either the gate and the gate circuit or between the gate and the emitter. This prevents an excessive increase in gate voltage and also prevents an excessive decrease in impedance of the IGBT.